ABSTRACT
Novel cobalt oxides, CaCo12O19 and BaCo12O19, have been synthesized under high-pressure and high-temperature conditions of 7 GPa and 1373 K, respectively. Rietveld refinement using synchrotron X-ray diffraction data indicates that the CaCo12O19 and BaCo12O19 crystallize in a magnetoplumbite structure with a hexagonal space group of P63/mmc (No. 194) as well as SrCo12O19. The magnetic study demonstrates that itinerant and localized 3d electrons coexist in all ACo12O19 (A = Ca, Sr, Ba) and the magnetic ground state transforms from antiferromagnetic (A = Ca) to ferrimagnetic (A = Sr) to antiferromagnetic (A = Ba), which is in stark contrast to the systematic change in the magnetoplumbite-related cobalt oxides of ACo6O11 from antiferromagnet (A = Ca) to ferrimagnet (A = Sr) to ferromagnet (A = Ba). The nonmonotonic magnetic evolution with isoelectronic A-site substitution in ACo12O19 is probably attributed to changes in the interactions between two magnetic sublattices of localized 3d electrons at trigonal-bipyramidal and tetrahedral sites for ACo12O19. This finding proposes the complex magnetic properties in the layered cobalt oxides with multiple magnetic sublattices in the coexistence system of itinerant and localized electrons.
ABSTRACT
A novel quadruple perovskite oxide CeCu3Co4O12 has been synthesized in high-pressure and high-temperature conditions of 12 GPa and 1273 K. Rietveld refinement of the synchrotron X-ray powder diffraction pattern reveals that this oxide crystallizes in a cubic quadruple perovskite structure with the 1:3-type ordering of Ce and Cu ions at the A-site. X-ray absorption spectroscopy analysis demonstrates the valence-state transitions in the ACu3Co4O12 series (A = Ca, Y, Ce) from Ca2+Cu3+3Co3.25+4O12 to Y3+Cu3+3Co3+4O12 to Ce4+Cu2.67+3Co3+4O12, where the electrons are doped in the order from B-site (Co3.25+ â Co3+) to A'-site (Cu3+ â Cu2.67+). This electron-doping sequence is in stark contrast to the typical B-site electron doping for simple ABO3-type perovskite and quadruple perovskites CaCu3B4O12 (B = V, Cr, Mn), further differing from the monotonical A'-site electron doping for Na1-xLaxMn3Ti4O12 and A'- and B-site electron doping for AMn3V4O12 (A = Na, Ca, La). The differences in the electron-doping sequences are interpreted by rigid-band models, proposing a wide variety of electronic states for the complex transition-metal oxides containing the multiple valence-variable ions.
